The use of linear and two-dimensional phased arrays of ultrasound transducers to achieve focus ultrasound beams is well known in the prior art. Such transducers employ a plurality of ultrasound-generating elements which are controlled to provide the ultrasound beam with a determined focal points. Such is achieved by control of the time of firing of the respective transducer elements. The scan of the ultrasound beam is accomplished by pulsing the ultrasound elements at different relative times to produce each beam (or "line"). Accordingly, as the relative timing of the energizations of the respective transducer elements changes, the azimuth taken by the beam moves in the direction of the scan.
Since the focusing of the ultrasound beam occurs through the control of the time of firing of the respective elements, the focus is predetermined at the time of firing of the elements. It is a characteristic of an ultrasound beam, at its focus, that a maximum level of ultrasound pressure is present. This feature is important when a contrast agent is utilized during the ultrasound imaging action.
The most widely used contrast agent comprises microbubbles that produce a strong nonlinear response when illuminated by an ultrasound beam. If the ultrasound beam exhibits, at its focus, an acoustic pressure which exceeds a threshold value, the contrast agent microbubbles can be burst. Use of this phenomenon during flow imaging enables a user to determine the time required to re-perfuse a flow region after the contrast agent therein has been destroyed by a high pressure ultrasound beam.
In current ultrasound systems, only one transmit focus is typically present per acoustic line or beam. To achieve the benefits of multiple transmit foci along a single scan direction, the prior art has used a "splice mode" wherein multiple acoustic lines are generated along the same azimuth, but with different focal depths as a result of altered timing of energizing pulses to the transducer elements. Data from the multiple co-linear acoustic lines are then combined to form a single composite image line with multiple foci. Other than the splice mode, a sector scan format used by current ultrasound systems produces focus loci which are semicircles that are centered at the apex of the scan.
The concentration of acoustic power at the focal point of an ultrasound beam provides a number of benefits. The echo signal-to-noise ratio in a region of interest is generally greater in echo signals received from the beam's focal point. Greater destruction of a contrast agent is achievable at the beam's focal point. This is especially useful in harmonic imaging methods that include subtraction of pre and post destruction image data.
As stated above, current ultrasound systems exhibit loci of focal points that are positioned along fixed, regular curves, e.g., in a sector scan, the locus of all beam foci are along a semicircle. There are, however, many anatomical features that present irregular outlines which are not aligned with the regular loci of focal points found in current ultrasound systems. For instance, when imaging cardiac walls, it would be desirable to control the loci of the ultrasound beam focal points to track the cardiac walls so as to achieve improved imaging thereof, or when contrast agent is being used, to enable improved its imaging or destruction.
Accordingly, due to the variability of anatomical structures, it would be useful to enable the user to define a path over which beam foci would track so as to enable an improved imaging of irregular anatomical structures.